As highly portable computers (such as notebook personal computers) are becoming increasingly common, front projectors have come into widespread use in meetings and presentations in offices to enlarge and display images created on computers. As video information delivery such as digital broadcast is achieved in a variety of ways with higher resolution, projectors have come into widespread use even in homes where images are viewed on a large screen. However, the use of a conventional projector with a large screen makes it difficult to obtain a sufficiently large projection space such as in small- and medium-sized meeting rooms or homes. In other words, unless a sufficiently large projection space is obtained, it is difficult for the conventional projector to project images onto a large screen.
One known way to reduce a projection space required outside a projector to provide a larger screen is to introduce a reflective surface into a projection optical system to fold, into the projection optical system, an optical path of a light beam used for forming a projected image. Another known way is to increase the angle of incidence of a light beam with respect to a screen surface to reduce the distance from a projection optical system to a screen. For example, in patent document 1, there is proposed a construction in which an optical path is folded by a reflective surface and this increases the angle of incidence of a light beam to reduce a projection space.
In the case of a front projector, where the projector and an observer are located on the same side with respect to a screen, the distance from a projection optical system to the screen is usually varied each time the projector is used. Thus, it is convenient to be able to adjust focus according to the distance from the projection optical system to the screen. In patent documents 2 and 3, there are proposed projection optical systems in which an optical path is folded by a reflective surface and this allows focus adjustment.    Patent document 1: JP-A-2004-258620    Patent document 2: JP-A-2006-184775    Patent document 3: JP-A-2005-106900